How to program a LPC 2420 microcontroller using a USB pendrive?

I am working on a project where we are using LPC 2420 in our product and earlier we were providing an option to upgrade the firmware of the product using serial connectivity (UART). But, now we want to provide an option to give the firmware upgrade through a USB pendrive which will contain the Hex file i.e. the firmware file.

Hardware configuration of USB is already done, but I am not able to find a solution as in how to proceed further with reading the file from the pendrive and reprogramming the external serial flash.

Additionally, it is also required to implement a bootloader. The application notes provide guidelines on the external memory access on LPC24xx, and a similar application that you are asking, but implemented on ColdFire and Kinetis microcontrollers: